Patent GB-1094400-A (GB)
Title: Television system and method
IPC: H04N3/00
Filed: 1965-07-23
Abstract: 1,094,400. Televison. STANDARD TELEPHONES & CABLES Ltd. July 23, 1965 [July 28, 1964], No. 31521/65. Heading H4F. In a television time-bandwidth compression system the line scanning of the camera tube, Fig. 4A, is controlled so as to proceed at a fast rate if the generated video signal assumes a level 89-1, 89-2, 89-3 etc. corresponding to white and to proceed at a normal rate when the signal assumes a level other than white, e.g. black, as at 90-1, 90-2, 90-3 etc. However, if a white level signal has a duration less than a predetermined interval T (measured by comparison with a pulse, Fig. 4C produced by a multivibrator triggered at the onset of each white level signal) the scanning is stopped, Fig. 4B, for an interval 95-1, 95-2, 95-3 etc. following the signal, the duration of the stop interval being such that when added to the white level signal the total duration bears the same relation to the white level signal as the fast scanning rate bears to the normal rate. Thus, in the example illustrated, where the fast rate is twice the normal, the stop interval equals the white level interval. Where the relation is 3 : 1 the stop interval is made twice the white level interval and so on for larger ratios. During the white level signal and following stop interval the video signal is caused to assume a level intermediate black and white. The final video signal may thus occupy three levels (apart from a " sync " level) as shown in Fig. 8A. Only the white signal 94-3 exceeds the duration T and proceeds at the level termed " fast white ", the remaining white signals assume the intermediate level. Due to the addition of the stop interval these remaining signals have durations the same as those which would have been provided had the related areas on the camera target been scanned at the normal rate. The intermediate signal level is termed " normal white ", indicating that the signal may be reproduced by scanning at normal speed. At a receiver for the system, the signal is detected at a first level 43, Fig. 8B, to produce a signal, Fig. 8C, for application to the modulator of a cathode-ray tube, and is also detected at a second level 50 to produce a signal, Fig. 8D, while controls the scanning to proceed at the fast rate during white signal 94-3 and at the normal rate during all other signals both black and white. The scanned line, Fig. 8E, thus duplicates the original image line at the camera, Fig. 8F. The overall system operation introduces a delay T. To ensure constant spacing between the raster lines for both camera and display cathode-ray tube the scanning rate changes are also applied to the field deflections. Although the waveforms shown in Figs. 4 and 8 are simplified in terms of an image containing only black and white areas, a grey scale may be accommodated by variations in the video signal, Fig. 8A, between black and normal white. As an alternative to the sync signal occupying a fourth signal level as shown in Fig. 8A, the signal may comprise a broad pulse occupying the signal range black to normal white which acts as pedestal for a centrally located narrow pulse occupying the signal range normal white to fast white. If such a signal is differentiated at a receiver it produces two positive-going voltage " spikes " followed by two negativegoing voltage " spikes ". Such a combination does not occur with any signal transition and may be readily distinguished. Suitable circuits for producing and separating such a sync signal are described with reference to Figs. 11-13 (not shown). Details of the video signal processing stages for the transmitter are described with reference to Figs. 3, 5, 6, 7, 14 and 15 (not shown) for a system where the fast scanning rate is twice normal rate and with reference to Figs. 16 and 17 (not shown) for a system where the fast scanning rate is three times normal rate. The invention may be applied to a normal television system based on a fast scanning rate or a slow scan system as described in Specification 1,051,931.